FIG. 1 is a circuit view of a conventional flat panel display. FIG. 2 is a schematic view of part “A” of FIG. 1, and FIG. 3 is a schematic view of part “B” of FIG. 1. A driving thin film transistor (TFT) 50 shown in FIG. 3 has a section as shown in FIGS. 4A and 4B. A flat panel display of FIG. 4A is an electroluminescent display and a flat panel display of FIG. 4B is a liquid crystal display. An electroluminescent display and a liquid crystal display have the same or similar driving TFT structure.
As shown in FIGS. 4A and 4B, semiconductor channels 80 serve as electrical connection channels for driving TFTs 50 and are formed on buffer layers 82 and 92 which cover substrates 81 and 91, respectively. When these semiconductor channels are derived from a polycrystalline semiconductor layer, an amorphous semiconductor layer 80a is crystallized into a polycrystalline semiconductor layer 80b by radiation of a laser beam 80c, as shown in FIG. 5. A crystallization process for a semiconductor layer may use a laser, i.e., an excimer laser annealing process is disclosed in U.S. Pat. No. 6,495,405. In this crystallization, in order to crystallize a wide amorphous semiconductor layer using a narrow laser beam, the laser beam shifts in a lateral direction of the amorphous semiconductor layer, i.e., in the direction of the arrow of FIG. 5 at a predetermined distance. In this case, a shift distance of the amorphous semiconductor layer is smaller than the width of the laser beam. Conventionally, the laser beam shifts at a distance corresponding to 1% to 15% of the width of the laser beam. In this regard, the overlap of the laser beam takes place in 80 to 99% of the amorphous semiconductor layer. When crystallization is carried out in this way, a region of the amorphous semiconductor layer is subjected to laser beam irradiation several times to several tens of times.
Stripes M are formed on a polycrystalline semiconductor layer obtained in this way, as shown in FIG. 6. Such stripes have characteristics different from the other areas of the polycrystalline semiconductor layer, and are hereinafter referred to as “heterogeneous straight lines.” These heterogeneous straight lines M may be caused by instability of the laser beam and uneven laser radiation in edge portions. These heterogeneous straight lines have different conductivity from the other areas of the polycrystalline semiconductor layer according to the operation condition of a driving TFT, thereby causing unevenness of luminance distribution of an image. This fact is demonstrated by a graph (in a 90% overlap condition) of a change of conductivity according to a distance in a semiconductor layer as illustrated in FIG. 7. As shown in FIG. 7, conductivity periodically varies depending on positions of the heterogeneous straight lines.
FIG. 8 shows a several column (L1) array of semiconductor channels formed in a conventional flat panel display regardless of positions of heterogeneous straight lines. In FIG. 8, while semiconductor channels with heterogeneous straight lines are represented by filled rectangles, semiconductor channels with no heterogeneous straight lines are represented by empty rectangles. In this regard, while some columns of semiconductor channels comprise no heterogeneous straight lines, the other columns of semiconductor channels comprise heterogeneous straight lines. Due to such an array of semiconductor channels, the semiconductor channels have different conductivities. For this reason, even though the same signal is supplied to the sub-pixels, there are luminance differences between light emission units of sub-pixels having driving TFTs with heterogeneous straight lines and light emission units of sub-pixels having driving TFTs with no heterogeneous straight lines. Therefore, there arises a problem in that stripes appear on a screen, thereby lowering image quality.